Most sonar systems include a single transducer lying many meters above the seafloor, to produce sonic beams resulting in sonic echoes from objects In the water or on the seafloor. By repeatedly transmitting sonic pulses and detecting the echoes as the sonar system moves, a technician seeks to detect changes indicating the presence of an object. Such systems generally cannot detect small objects lying at or under the seafloor, or provide detailed information as to the characteristics of the object such as its size, its density profile, etc. My earlier U.S. Pat. No. 4,924,449 describes a method for probing the seafloor, which includes a stationary platform and a transducer that can be moved to any one of several different locations on the stationary platform. At each location, the transducer generates a sonic beam and detects the echo from that beam, before being moved to another location. While this approach enables detection of seabed characteristics at several specific locations, it does not enable the rapid generation of a display that enables a person to visualize characteristics of a seafloor area so as to pick out areas of interest or to obtain an understanding of the vitality for flora and fauna, of a seabed area. A system for probing a seabed, which created signals and usually a display, indicating fine details of a seabed, would enable a better assessment of the quality and unique characteristics of a seabed and better detection of areas of interest, which are usually man-made and geological objects of interest, such as breaks in pipelines and cables, buried mines, and buried boulders.
In accordance with one embodiment of the present invention, an apparatus and method are providing for evaluating regions at and under a seafloor, which provides signals representing seafloor characteristics along numerous closely spaced primarily vertical lines. The apparatus includes a linear array of closely spaced (preferably spaced no more than 25 cm apart) transducer devices that is moved perpendicular to the line of the array to create the equivalent of an X-Y array, or includes an actual X-Y array. Each transducer device is briefly energized to produce a narrow sonic beam that directed at a small area of the seabed. Echoes from that beam are detected, and thereafter another transducer is energized to produce a sonic beam that is directed on an adjacent area of the seabed, etc.
The apparatus can use the echo signals to generate a display having three axes, including numerous lines extending along parallel planes, where each line represents an echo from a single pulse of a single transducer of an array. A second axis can represent different depths or different frequencies at a particular depth. A third axis that is largely vertical can represent the overall amplitude of the echo at different depths or the amplitude of different echo frequencies at a particular depth within the seafloor. The display of echoes from the transducers of the array enables a person to detect anomalies that may indicate objects or the condition of areas of interest.
Each sonic pulse includes a carrier of high frequency that is preferably at least 200 kHz, with each pulse length being on the order of magnitude of 10 microseconds, and with each sonic pulse having a peak energy of about 0.5 to 5 wafts per square centimeter of transducer face. The high frequency carrier produces a narrow sonic beam, the duration of the pulse determines the center frequency of the frequency band produced by the high energy pulse passing through water, while the high energy of the pulse creates a transformation to a wide frequency band by water. The echoes cane be automatically analyzed to detect anomalies, as by detecting a large change in amplitude (of all frequencies combined) with vertical depth or horizontal position, or to detect a large change in amplitude at a particular frequency of closely spaced locations in the seabed. A detection of large amplitude at a particular frequency or large changes in amplitude at a particular frequency with small changes in depth or horizontal position, can be used to change the duration of the pulses produced by the transducers, so more energy of a frequency of interest (narrowed bandwidth) is produced.